I have an inground gunite pool that is several years old. The pool water is perfectally clear but the walls and bottom are stained green. The free chlorine is good, really high right now, The PH had been low so I raised it. Went to pool supply company and asked them about greenis yellow walls and bottom, they told me that Black Alge out would be best chemical as Black Alge is most aggressive and this should take care of the problem. I have done two treatments with the blsack Alge out and the water is the clearest it has ever been but the walls and bottom are still stained. the Green won't even brush off as the instructions on the black Alge out said to do. Never had the problem this long before. When I had it before a strong dose of chlorine would take care of the problem. At wits end.

JTS, I posted a message about a month ago with no replies. I have not figured it out sorry to say but we may have the same problem so I'll keep you up to date and please do the same.

Here is the message I posted, tell me if this is familiar.

" I have an in ground '20 x '40 120000 liter 36000 gallon pool with an AquaRite salt system.Last year I started to notice green patches on the liner. This year much of the bottom is stained. The green stain is neither rough nor slimy. It does not come of with an aggressive brushing however I have noticed that if I drop a rock of salt on a patch of green it will lighten. Last year the pool was run with the water temperature quite high all summer and there may have been inadequate chlorine. The readings were in the norm all summer but I have since read that the chlorine level should have been a bit higher with the warm water. Perhaps this was a contributing factor.Last week I brought the PH down with PH-, shocked 1 hour later then added HTH algaecide 60 an hour after that. That did not help. Today I tried using three times the usual amount of shock and so far nothing has changed.I have not tried acid. I am thinking that the chlorine should have killed and gotten rid of anything already. Is it possible that what I have is not algae?None of the pool stores nearby have a solution. They keep trying to sell me product after product."